Molecular decomposition reactions and early nucleation steps in CVD growth of graphene on Cu and Si substrates using toluene

Graphene growth from chemical vapor deposition (CVD) commonly employs methane (CH₄) as carbon precursor but requires temperatures in excess of 900°C. Aromatic hydrocarbons, especially toluene (C₇H₈), may lower the growth temperatures well below 600°C, while preserving graphene quality; however, molecular decomposition reactions and early nucleation steps of CVD graphene using toluene are not known in details. We investigate the decomposition steps of toluene adsorbed onto Cu(111) and c(4x2)-reconstructed Si(100) surfaces through DFT calculations. The geometry and energy of toluene and most likely decomposition by-products were analyzed for various adsorbate structural configurations. Early decomposition reactions were studied through investigation of minimum energy pathways and transition states. Low activation energies were found for H removal from the methyl group of toluene physisorbed on Cu(111) (1.20 eV) or chemisorbed on Si(100) (1.39 eV), leading to the formation of benzyl radicals; further dehydrogenation reactions of the latter lead to C₇H₆, C₇H₅ and C₇H₄ fragments, their formation being energetically feasible on Cu (energy barriers in the 0.87-1.62 eV range) but not on Si. These radicals may act as active species in the formation of sp²-bonded carbon nuclei during CVD growth of graphene. Anthracene (C₁₄H₁₀) formation from two closeby C₇H₅ radicals has been studied through meta-dynamics and umbrella sampling methods applied to molecular dynamics simulations. Preliminary results indicate that zig-zag anthracene could easily form onto Cu(111), the energy barriers being below 1.1 eV. A prohibitive energy was instead obtained for (100)Si, hindering anthracene formation on this substrate under practical CVD conditions.